1. Field of the Invention
This invention relates generally to a method and apparatus for determining the size of a borehole and, more particularly, to techniques for processing borehole size measurements obtained with downhole sensors to determine the borehole diameter. The invention has general application in subsurface exploration and production, but is particularly useful in while-drilling operations.
2. Description of Related Art
In order to improve oil, gas, and water drilling and production operations, it is necessary to gather as much information as possible on the properties of the underground earth formations as well as the environment in which drilling takes place. Such properties include characteristics of the earth formations traversed by a well borehole and data on the size and configuration of the borehole itself. Among the characteristics of the earth formation of interest to drillers and petrophysicists is the resistivity of the rock or strata surrounding the borehole. However, the processes often employed to measure these characteristics are subject to significant errors unless information on the borehole size and configuration is also taken into account in their determination. Knowledge of the borehole size is also useful to estimate the hole volume, which, in turn, is used to estimate the volume of cement needed for setting casing or when hole stability is of concern during drilling.
The collection of downhole information, also referred to as logging, is realized in different ways. A well tool, comprising sources and sensors for measuring various parameters, can be lowered into the borehole on the end of a cable, or wireline. The cable, which is attached to some sort of mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
An improvement over wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance and avoid financial injury due to well damage such as collapse or fluid loss. Formation information collected during drilling also tends to be less affected by the drilling fluid (xe2x80x9cdrilling mudxe2x80x9d) invasion processes or other undesirable influences as a result of borehole penetration, and therefore are closer to the properties of the virgin formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are know as logging-while-drilling (LWD). However, the terms MWD and LWD are often used interchangeably, and use of either term herein includes both the collection of formation and borehole information, as well as data on movement of the drilling assembly.
It is known in the art to measure the diameter, also known as the caliper, of a borehole to correct formation measurements that are sensitive to size or standoff. These corrections are necessary for accurate formation evaluation. U.S. Pat. No. 4,407,157 describes a technique for measuring a borehole caliper by incorporating a mechanical apparatus with extending contact arms that are forced against the sidewall of the borehole. This technique has practical limitations. In order to insert the apparatus in the borehole, the drillstring must be removed, resulting in additional cost and downtime for the driller. Such mechanical apparatus are also limited in the range of diameter measurement they provide.
Due to the unsuitability of mechanical calipers to drilling operations, indirect techniques of determining borehole calipers have been proposed for LWD measurements. Conventional LWD caliper measurement techniques include acoustic transducers that transmit ultrasonic signals for detection by appropriate sensors. U.S. Pat. Nos. 5,469,736 and 4,661,933 describe apparatus for measuring the caliper of a borehole by transmitting ultrasonic signals during drilling operations. U.S. Pat. No. 5,397,893 describes a method for analyzing formation data from a MWD tool incorporating an acoustic caliper. U.S. Pat. No. 5,886,303 describes a logging tool including an acoustic transmitter for obtaining the borehole caliper while drilling. U.S. Pat. No. 5,737,277 describes a method for determining the borehole geometry by processing data obtained by acoustic logging.
U.S. Pat. No. 4,899,112 describes a technique for determining a borehole caliper by computing phase differences and attenuation levels from electromagnetic measurements. U.S. Pat. No. 5,900,733 discloses a technique for determining borehole diameters by examining the phase shift, phase average, and attenuation of signals from multiple transmitter and receiver locations via electromagnetic wave propagation. GB 2187354 A and U.S. Pat. No. 5,519,668 also describe while-drilling methods for determining a borehole size using electromagnetic signals.
U.S. Pat. No. 5,091,644 describes a method for obtaining a borehole size measurement as a by-product of a rotational density measurement while drilling. U.S. Pat. No. 5,767,510 describes a borehole invariant porosity measurement that corrects for variations in borehole size. U.S. Pat. No. 4,916,400 describes a method for determining the borehole size as part of a while-drilling standoff measurement. U.S. Pat. No. 6,285,026 describes a LWD technique for determining the borehole diameter through neutron porosity measurements.
All of these subsurface measurement techniques are influenced by their immediate environment, and this influence has to be corrected to obtain an accurate measure of the undisturbed formation and borehole geometry. Thus it is desirable to obtain a simplified method for accurately determining the borehole shape and size. Still further, it is desired to implement a borehole size measurement technique that works for a wide range of borehole sizes and offers flexibility of measurement modes.
The invention provides a method for determining the size of a borehole penetrating an earth formation. The method comprises obtaining a plurality of borehole size measurements, each said measurement derived from one of a plurality of sensors that were disposed within said borehole; weighting each borehole size measurement with a factor associated with said measurement; and processing said weighted measurements to determine the borehole size.
The invention provides another method for determining the size of a borehole penetrating an earth formation. The method comprises obtaining a plurality of borehole size measurements derived from a plurality of sensors that were disposed within the borehole, said sensors being adapted to make said measurements using different measurement principals; determining a set of mathematical equations representative of the responses of said plurality of sensors; and solving said equation set to determine the borehole size.
The invention also provides a computer encoded with instructions for performing operations on a plurality of borehole size measurement inputs acquired with a plurality of sensors that were disposed within a borehole traversing a subsurface formation, the sensors being adapted to make said measurements using different measurement principals. The instructions comprise weighting each input with a factor associated with said measurement; and iteratively processing said weighted inputs to determine the size of said borehole.